The fierce crocodile that eats dirt

With an estimated 100 years of life left in its open pit, Anglo American Platinum’s Mogalakwena Mine in the Limpopo Province is destined to remain a top platinum producer for many years to come, writes Leon Louw, editor of the magazines Mining Mirror and African Mining.

About 20km from the town of Mokopane on the way to the Botswana border, an isolated steel structure has sprung up among the granite hills and clutches of sickle bush and camel thorn trees. The structure belongs to Canadian business mogul Robert Friedland’s Ivanhoe, and it indicates where Friedland has recently sunk a new shaft into the Northern Limb of the Bushveld Igneous Complex’s platinum reef. The headgear is part of Friedland’s Platreef platinum project, which is still a work in progress. Nevertheless, its presence is significant. Underneath the headgear lies what Friedland has described as ‘the greatest platinum discovery the world has ever seen’. His claim is hard to believe when staring at the headgear standing on its own in the red brown dust of Limpopo. But it’s true, and becomes more believable when driving on for another 10 minutes or so before turning left off the main road, where suddenly one is confronted with massive waste dumps, two big tailings dams, and an imposing processing plant. This is real mining territory. The land belongs to Anglo American Platinum (Anglo) and is known by its workers as the Republic of Mogalakwena. The mine that is responsible for the massive pits and dumps is Anglo’s Mogalakwena platinum mine, which in Pedi translates to the “fierce crocodile”.

Lucrative surface mining

Anglo mines the same ore body at Mogalakwena that neighbours Ivanhoe intends doing at Platreef. The difference, however, is that Mogalakwena has been around for much longer, and its platinum reef is found very close to the surface, which makes opencast mining extremely lucrative. Friedland, on the other hand, will have to go digging a lot deeper, and has decided to spend more capital upfront, sink a shaft, and access the reefs via underground mining methods. Nonetheless, the one thing that both companies have is access to the best platinum ore body in the world. Although much higher grades are found at other platinum deposits in South Africa, the fact that especially Mogalakwena’s reefs are so shallow makes this geological occurrence unique. According to Hermann Hollhumer, senior engineering manager at Mogalakwena, the other difference between the Northern Limb (where Mogalakwena is located) and the Eastern and Western limbs of the Bushveld Complex is that the Northern Limb’s reef is between 50m and 200m thick and dips at about 45 degrees. The entire Northern Limb is about 100km long and varying degrees of minerology occur throughout its extent. The Eastern and Western limbs also host UG2 and Merensky ore, but the reefs are very narrow and the Eastern Limb dips between 10 and 16 degrees. The reef outcrops at a number of locations in the east and west as well, but the difference is that the reefs close to surface are (at most) only 2m thick, compared to the 20–100m on the Northern Limb. In addition to the platinum group metals (PGMs), which most mining companies on the Western and Eastern limbs mine, Mogalakwena’s basket of minerals is larger and it processes metals like copper, nickel and gold as well.

The Northern Limb’s treasure chest

Hans Merensky first discovered the platinum deposits of the Northern Limb in 1925. Tentative underground tunnels were dug in the mid-1900s, but serious mining only started in 1993 when a company called Potgietersrus Platinum started an opencast operation where Mogalakwena is located today. The mine was taken over by Johannesburg Consolidated Investments (JCI) before being transferred to Anglo American in 1995. The original pit, called Sandsloot, was about 400m in diameter. Today, Anglo unearths material from five different pits (Sandsloot, Zwartfontein, Central, South and North), and according to Hollhumer, the company actively mines about 10km of land.

At a time when low commodity prices have been wreaking havoc on the markets and Anglo has been selling off a number of its prime assets — including its platinum shafts in the Rustenburg area — Mogalakwena has been going from strength to strength. It is now Anglo’s most valuable mine, and the fact that it wasn’t listed as a possible sell on Anglo American group CEO Mark Cutifani’s list of things to do, reiterates Cutifani’s unwavering belief in Mogalakwena as a money spinner. It can indeed be regarded as a diamond in Anglo’s next bag of tricks. The mine’s greatest advantage is that the ore body is simple and shallow and therefore lends itself to opencast and mechanised mining. This is in total contrast to the complex and narrow reef Rustenburg shafts, some that incidentally made it to the top of Cutifani’s list of dogs to be sold.

Mogalakwena currently mines about 100 million tonnes of material straight from its five pits, of which it mills close to 12 million tonnes of full grade ore at a little bit more than three grammes per tonne (g/t) of platinum. “This year we will produce more than 400 000 ounces. In 2012 we delivered 300 000 ounces. With little capital the Mogalakwena team has managed to improve efficiencies and effectiveness, and increase output by more than 30%,” says Hollhumer. The mine feeds three concentrators: the north and south concentrators are both located on site, while ore is also sent to the Boabab concentrator that Anglo rents from Lonmin. Boabab is located close to Polokwane, about 100km from Mogalakwena.

Improvements through innovation

Much of the success that Mogalakwena has achieved can be ascribed to good management. Innovation and technology is a big focus, and the mine continuously aims to improve efficiencies. The mine runs an ultra-class fleet of mining equipment. The most unique feature in the pit is probably the Chinese manufactured TZ WK-55 rope shovel. This monster has a 100-tonne payload, and according to Hollhumer it is the most cost effective rope shovel in the western world. Although it was risky to acquire a brand that hasn’t been used in an Anglo pit before, Hollhumer says that a technical team of about 10 people conducted rigorous tests on the machine for almost three months before the rope shovel was finally purchased. “The machine has been running without problems for close to three years and has given us excellent service,” says Hollhumer. The machine, part of a joint venture between VR Steel and TZ China, is maintained and supported locally. Other ultra-class equipment in the pit includes 21 Komatsu 930E rigid dump trucks (RDTs) with a 300-tonne payload — a fleet that (according to Hollhumer) benchmarks globally in the top 90% in terms of performance. The mine further boasts ten 4400 Terex UnitRigs.

Working the pit

In the pit, Mogalakwena does all the blasting pattern preparation themselves. This process includes front-end loader cleaning, excavator cleaning, and track dozer levelling. The pattern is then drilled by using a GPS based drilling system to ensure high quality drilling. The benches in the pit are 15m deep. Once the drilling is done, drill and blast contractors AEL and BME carry out the blasting. After the blast, either the TZ rope shovel or the hydraulic shovels load the material. “We have seven O & K CAT hydraulic shovels. One of them, the RH400, is the biggest hydraulic shovel in the world and is the only electrically driven unit in the world,” says Hollhumer.

The rope shovel loads about 100 tonnes per pass and per shovel load. The RH400 has a capacity of 80 tonnes, while the RH340s load approximately 60 tonnes per pass. The RDTs spend two minutes at the shovels while being loaded. A three pass loading design should enable the operator to load onto a truck every 30 seconds. When using a hydraulic shovel the operator will manage five or six passes, in which case it is a little bit longer — about two minutes on average to fill a truck. The cycle time is roughly 35 minutes; in other words, it takes 35 minutes to load, drive to the dumping point, discharge and come back. One RDT will typically do two loads per hour. The RDTs are governed at a speed of 45km per hour and operate on a 10-degree incline. The strip ratio in the Mogalakwena pit is five to one, and this figure is continually increasing as the pit gets deeper. This means that for every five tonnes of material mined, the mine extracts one tonne of ore.

Electric benefits

According to Colin Ott, new mining technology project manager at Mogalakwena, the benefits of using electric rope shovels are myriad. “Electrical power is three times cheaper than diesel power. Most mines use diesel applications because it is more nimble and easier to move around the pit; however, there is a significant energy benefit in using electric shovels,” says Ott. A significant drawback of using electric equipment in the pit is that equipment needs to be moved around continuously. However, Hollhumer says that this challenge can be overcome through thorough and pro-active planning to move transformers and motivators at the right time to the right place (a motivator is a power plant on wheels). “A shovel shouldn’t be moving every day, but once a week or once every second week,” says Ott.

From stockpiles to concentrators

After the shovels have loaded the material into the dump trucks, the trucks deliver it to either the waste dump (85% of the mines output) or to the ore stockpiles. “We stockpile by grade into very low, medium and high grade ore, and have a blending strategy that ensures we deliver to the plant at about 3.1g/t,” says Hollhumer. The ore is transported to one of the abovementioned three concentrators where one metre of rock is reduced to 53 microns, after which chemicals are added and platinum extracted. A number of features on the Mogalakwena plant are different from traditional platinum plants, and according to Hollhumer the plant has a 77% recovery rate. “We have a high pressure grinding roll (HPGR): a machine more common in diamond mines, but definitely unique in the platinum industry. Our process starts with a gyratory crusher; then secondary or high-pressure crushers (Mogalakwena uses Metso, FLS and Sandvik crushers); followed by the HPGR (Thyssenkrupp and FLS); then primary and secondary mills; tertiary or SA mills (FLS, Metso, Outotec, Thyssenkrupp); then the float, thickeners and filtration, before the concentrate is dispatched to the smelters. Anglo’s smelters are located in Polokwane, Rustenburg and Northam. According to Hollhumer, Mogalakwena dispatches between 1 000 and 2 000 tonnes of concentrate per day — that is about 10 trucks per hour. All the concentrate is transported to the smelters by road.

Drilling beyond the faults

Although Hollhumer says they are aware of two main faults in the pit, mine planning happens around these faults and it doesn’t really affect the mining or production. “Our biggest opportunity at Mogalakwena involves stakeholder relationships. However, the operation is in a good space,” he adds. Mogalakwena invests a lot of time and money in better technology. One of its main values is innovation and continually improving processes and systems. The current big focus for Ott and his technology team at the mine is the commissioning of a new autonomous drill rig. Ott’s team is working with original equipment manufacturers on a machine that will be able to drill within five or six metres from the highwall in the pit, without an operator. “Apart from the safety benefits of an autonomous drill rig, continuous cost benefits exist. The drilling is more optimised, more fuel-efficient, and requires less maintenance. An autonomous system gives the team a better, more consistent penetration rate. So over a period of time you will gradually realise a much bigger benefit,” says Ott.

Mogalakwena has nine Atlas Copco electrically driven Pit Viper drilling rigs, four Atlas Copco DML rigs, and three smaller Atlas Copco D65s. The DMLs and D65s are diesel powered. In addition to the autonomous drilling system, the technology team is also currently implementing and installing collision avoidance systems on all their light duty vehicles and dump trucks. They are also looking at a trolley system that will enable the large RDTs to be electrically driven up slopes on the haul roads, while converting back to diesel on flat surfaces. Mogalakwena is one of the best run opencast platinum mines in the world, and could remain so for the next 100 years, which is the estimated life of the pit. “Then,” says Hollhumer, “We might start looking at mining the extensive reserve with underground mining methods.”